Photographic emulsion and process of producing it



Patented Sept. 23, 1941 PHOTOGRAPHIC EMULSION AND PROCESS OF PRODUCING IT Friedrich Lierg, Berlin-Zehlendorf, Germany, as-

signor of one-half to Oskar Czeija, Wien IV,

Germany No Drawing. Application December 5, 1938 Serial 1937 In Austria December 6,

16 Claims. ((31. 95-7) This invention relates to photographic emulsions and to a process of producing the same.

Photographic emulsions are suspensions of a silver halide in a protective colloidal medium.

They are produced by setting up a reaction within the colloidal medium between the salts of a halogen and a salt of silver, whereby highly dispersed silve: h ide is produced. Emulsions consisting of the protective colloid and the silver halide are of extremely fine grain, sometimes even designated as grainless. But in this form they are very little photo-sensitive and must be subjected to a ripening process in a known way. The ripened emulsions Whose photo-sensitiveness is substantially higher contain the silver halide in the form of more or less coarse granules maintained colloid. Sensitiveness and size of grain are interrelated in some manner. Negative emulsions of great sensitiveness have coarser granules, negative emulsions of less sensitiveness may have finer grain. Many attempts were made to reduce the coarseness of the grain, as fine grain is desirable for many purposes, or to eliminate the granular structure entirely, but it is only within the last few years that fine grained emulsions of high sensitiveness have been produced.

The present invention deviates from the methods heretofore practiced by being based on the introduction of silver halide into a liquid carrier colloid, producing thereby a photo-sensitive emulsion which is ready for pouring, since it is grainless; at the same time it has great sensitiveness to light, is not decomposed by the de-' veloper and does not require ripening or repeated washing operations.

When two colloids are mixed with each other, the character of the mixture varies depending upon the charge of the'colloidal particles.

(1) When the two colloids carry charges of the same polarity, a protective effect is produced in 'most instances, in such manner that the colloid of greater stability stabilizes the second colloid against coagulating electrolytes, sedimentation,

in suspension by the protective larity, respectively, generally precipitate each other. It is the charge on the particles which produces the stabilizing efiect, and this eiiect is destroyed by neutralization. Two colloids produced by mutual precipitation are very firmly stable, and flocculation occurs. If, however, oneof the colloids is present in excess as to quantity compared with the other colloid, or has a charge of excessive strength as compared with the opposite charge in the other colloid, a prevailing charge will be noticeable after the mutual precipitation has taken effect, andthis end charge or resulting charge will be sufficient to stabilize the'combination. But in that case now, the union between the two colloids remains a. very thorough and firm union, so that even heat treatment (for instance) will not lead to an enlargement of the particles.

Photographic emulsions in use up to now are produced in accordance with the first described process relying on so-called protective colloids. The silver bromide is charged negatively owing to the' excess of gromium salt employed for producing the emulsion. The carrier colloid in almost all instances is gelatine, and this gelatine also is charged negatively eventhoughthe charge is weak. Hence, this protective colloid protects the silver bromide against dlsassociation, although ordinarily the colloidal silver bromide would be stable for short time only.

The present invention is based on the principle 1 set forth above for the alternative process of producing photo-sensitive emulsions from colloids with opposite charges.

According to this invention, a silver halide is introduced into a carrier colloid having the opposite charge and is retained therein by adsorption. The quantities and the charges of the two ingredients must be so related that flocculation sion is subjected in warm condition to digestion for a longer period, the particles will coagulate and finally sedimentation will occur, in spite of the protective action.

(2) Colloids having charges of opposite powill not occur, and that the product has a resultin'g'lor remaining charge diflerent from zero, sufllclent to stabilize the compound as required for the desired photographic qualities.

It is an object of the invention to provide a process in which all" conditions for a stable union of the colloids are met with respect to the selecthe present process may be derivatives of cellulose or synthetic products produced by polymerization or condensation or by both, and similar materials or mixtures of materials.

It is essential that the particles of the carrier colloids should be charged sufiiciently to assure that even in emulsions which are rich in silver halide the resulting final or remaining charge will suflice to produce stability.

Hence it is also an object of the present invention to utilize the phenomena of colloidal chemistry for enhancing a charge of the carrier colloid by a suitable treatment of the colloid pro: vided the original charge of the same should not be. strong enough. If, for instance, in certain colloids the charge of the particles is due to the I presence of certain active groups in the molecule, it may become of importance to increase the number of these groups in order to strengthen the charges of the particles. For instance, in cellulose derivatives in which the value of the charge with respect to aqueous solvents depends upon the number of the free OH-groups, the efliciency of the carrier colloid may be enhanced by increasing the number of these OH-groups. Several methods maybe employed for this purpose. As an example it may be mentioned that in the production of cellulose ethers, a part only of the OH-groups may be etherized- Also with cellulose esters, the desired prevailing quantity of free OH -groups is obtained in the same way or possibly also by partial saponification of the cellulose esters.

Colloids whose charge may be varied due to the adsorption of other ions are rendered suitable for the present process by being treated with substances which impart to these colloids the desired charge. Many colloids, for instance, are charged negatively by means of alkalies, or the alkalies may have the effect of enhancing the negative charge originally present in these colloids. A treatment with acid, on the otherhand, instead of with alkalies may be used when a positively charged colloid is to be employed for the preparation of the emulsion-in which case, owing to the adsorption of hydrogen ions, the positive charge will be increased.

The synthetic products which may advisably be used" are products which have the active groups required for charging the particles and in which these groups are'producedby the meth-- od by which the products are prepared. This includes, for instance, the polymerized acrylic acid derivatives or condensates of vinyl alcohol and the like. If necessary, the charge of the synthetic products may be controlled inthe manner described above. The charge in the silver halide as required for the present process also is created by the manner in which the halide is produced. In the presence of excessive quantities of silver ions, a positively charged silver halide may be prepared. In the presence of excessive halogen ions, a preparation having a, negative charge will result.

The production of the silver halide may, therefore, be efiected in the following way:

Assuming that the carrier colloid is negatively tions.

other hand, the carrier colloid is positively' charged and is to be united with negative silver halide, there must be present an excess of the halogen compound over the silver compound.

Furthermore, it is essential that upon completion of the production of silver halide, there should be present an excess either of the halogen or of the silver respectively, determining the character of the charge. In preparing, therefore, a positively charged silver halide, it is necessary to combine with the silver a quantity of the halogen salt less than what would be required stoichiometrically, and if a negatively charged silver halide is to be produced, thequantity of the silver compounds must be less than computed stoichiometrically with respect to the halogen salt.

The two ingredients of the silver halide, namely, the silver compound and the halogen compound may be introduced into the colloid carrier in various ways. The entire quantity of one compound, namely, the silver compound or the halogen compoundmay be added to the colloid carrier, and then the required quantity of the respective other compound may be introduced. Instead of introducing the two compounds each in a separate single operation,. the two compounds may each be introduced in several por- The two compounds may be introduced over the same period of time, in which case the compound required in excess would have to be introduced at a greater rate of speed. It is also feasible to introduce one or both compounds in a portion only of the colloidal carrier and to complete the formation of the silver halide in this manner. The manner in which the reaction itself is induced, therefore, is not as much of importance as the fact that there should always be an excess of one of these ingredients: an excess of the silver salt when it is desired to produce a positively charged silverhalide, or an excess of the halogen salt when the silver halide is to be charged negatively.

Silver nitrate is generally employed as a silver salt, but obviously other compounds of silver may be used. I

The solvents also must be selected in accordance with the objects of the present invention. The solutionsof colloids are not true solutions but are formed of mixtures or micells'which have a surface charge with-respect to the solvent. Now, the charge of 'thissolution might have the efiect of depressing the polarity of the. colloid carrier charge. On the other hand, the charge produced in the developing process. The solvents to be employed, therefore, must be solvents which do not interfere with the adsorbing efl'ect produced by the colloid carrier on the silver halide, and no interfering deceptive polarity must be created. In using as carrier colloidal acetyl cellulose which has a negative charge, the following solvents may advisably be employed: methyl alcohol and other alcohols, acetone, chlorinated hydrocarbons, diacetone alcohol and other solvents or mixtures of such solvents, with organic solvents and blending agents. But also aqueous mixtures of the same may be used as for instance acetone and water.

Upon introducing silver halide into a carrier colloid having. a strong opposite charge, the resulting or finally prevailing charge is utilized to control the photo-sensitiveness of the emulsions produced in the present process. The photosensitiveness of emulsions is dependent upon certain factors. Different emulsions in which these factors are present in the same proportions will, however, be more sensitive if the resulting, or remaining charge is small. It is, therefore, necessary or advisable to reduce this resulting, ultimately prevailing charge as much as possible. This reduction of the strength of this charge is effected by making the treatment of the colloidal carrier with the oppositely charged silver halide as thorough as possible, or by regulating the ultimately prevailing charge through the addition of other ingredients to the combination, adapted to reduce the charge. If the ultimate charge of the colloid carrier is, for instance, positive it may be reduced or neutralized, so to speak;

with acidic dyes, colloidal metals and other similar substances, while negative ultimate charges may be reduced or neutralized by means of colloidal oxides or hydroxides, basic dyes or similarly acting substances. I

It has been stated above that for the production of emulsions from a colloid carrier with negative charges and asilver halide with positive charges an excess of the silver salt is required which excess is adsorptively bound to the colloid carrier. By suitably regulating this excess over the quantity of silver salt which'is required for producing the emulsion, the quality of the pictures to be produced may be controlled. In a general way it may be stated that the gradations of the picture become less rich in contrast and softer if the excess is small only. Various methods may be used for varying the excess which controls the gradation. So for instance, it is feasible to provide for the desired excess even when preparing the entire emulsion. It is more advisable, however, first to produce the emulsion and then add the excess quantity of one of the ingredients. Other methods of adjusting the excess value permitting the finished emulsion to be varied include washing operations, as for instance, dialysis, or they include precipitation, as for instance, by means of cobalt chloride. It is also feasible to partially substitute for a portion of the silver salts some compounds of metals which change the charge of the silver halide to a positive charge or render it more positive than it had been before. Compounds of thallium'may for instance be used for this purpose. These correcting or adjusting compounds may, however, also be added right during the production of the emulsion.

It is characteristic of the emulsions made up of a negatively charged colloid carrier and positively charged silver halide that they fail to produce a general halo in the developer in spite of their contents of silver salt. This advantageous behavior of the emulsion inthe developer is due to the fact that the silver salt is adsorptively bound on the colloid carrier.

The emulsions of this character are, furthermore, distinguished by great stability in respect of electrolytes, due to the firm union of the colloid carrier with the silver halide. This stability makes it possible for many purposes to leave the reaction-salts right within the emulsions eliminating the laborious washing operations otherwise required with gelatine emulsions. It is'particularly to be noted, however, that the emulsions produced by the present process are ready for use without requiring any ripening.

The emulsions produced by this process may be poured out andused directly. They may, however, also be applied as'permanent coatings 'to any desired supports or carriers, as glass, metal, paper, films made from Celluloid, fabrics of acetyl cellulose and the like. The emulsion may be poured on these carriers in a single layer or a plurality of layers, and if desired, filter layers or other intermediate layers may be applied without difficulty. Upon pouring the emulsion upon the carrier, it should be noted that solvents must be used, which must have no dissolving efiect at all or a very slight solving effect only upon the carrier or upon any intermediate coating respectively. It is advisable to cement the emulsion to the carrier or to the intermediate coatings, filter layers,'etc. by adding a solvent of high boiling point to them.

The use of this process is accompanied by considerable advantages andfurnishes emulsions of highly desirable qualities.

Being of non-granular structure, the emulsions furnish a photographic material, for negatlves or positives, which permits enlargements practically without any limits. The absence of the granular structure produces in the pictures a sharpness of contours and a merger of different degrees of contrast which excel all known emulsions, including even collodion emulsions which are considered the best available emulsions for process purposes. The material is, therefore, suitable for this purpose in all graphic arts and is of great value also for the production of tone-true sound records.

Photographic coatings made of such emulsions cannot cause any dispersion of light in the absence of granular particles. to a higher degree than heretofore to produce a coating from a plurality of layers, as required for instance for color films. Photographic coatings made of these emulsions show a far better covering power as compared with usual grain emulsions of halide silver having the same silver contents. This is ofdm'portance for reproduction processes, for it makes it possible to employ thinner coatings which will produce sharper prints.

If the charge of the colloid carrier particles is made sufllciently high, it is feasible to introduce far greater quantities 'of silver halide into these colloid carriers than in emulsions having the ordinary protective colloid only. In this manner Hence, it is feasible dyes which are adsorbed on the surface of the 1 silver halide. Where the colloid carrier has a 1 strong negative charge, a certain quantity of the silver salt may be added to the emulsion as a halogen accepter, without risking that'this silver 1 salt will be reduced in the developer to form a 1 halo, The positively charged silver ions are adsorbed by the negative colloid. The production of the improved emulsions is greatly simplified owing to the elimination of the ripening operation. It has the advantage over 3 the production of emulsions in protective colloids that the qualities of the final product can be predetermined with great accuracy, as 'conmerated in the following given by way of example only:

One hundred gram acetyl cellulose having an acetic acid content of 54 per cent, are dissolved in 800 cubic centimeters acetone. Three hundred cubic centimeters water are added, and while stirring, 30 cubic centimeters alkali hydroxide of OH-groups which had been esterized. This leads to the production of acetyl cellulose having an acetic acid content of about 36 per cent., but

1 improved emulsions are not subject to self-rip- I ening and they, therefore, can be preserved in 1 liquid condition without altering their qualities. It is not necessary to store them in low temf perature rooms and .to pour them solely when they have;reached a certain stage-.-a condition "which in protective colloid emulsions always had 1 to be ascertained by pouring tests. The fact that the washing operation may be dispensed with has already been mentioned above. The elimination of this laborious and costly washing op- 1 eration renders the entire production much more ticles to be influenced, as by the present process ,acetyl cellulose is ordinarily not adapted as a carrier for silver halide. (See Ullmann, Encyclopadie der Technischen Chemie" vol. I, page j 138, third paragraph.)

United States Patent No. 2,110,491 describes a process of producing photo-sensitive emulsions by employing acetyl cellulose which had been '1 highly hydrolyzed. Acetyl cellulose of this typei would ordinarily'show a negative charge of the particles. But this prior patent enumeratesin its Examples 1 to 3 some steps of forming silver halide entirely different from those specifically required for the present invention.

The prior patent also. refers in Examples 2 and i 3 to an excess of the halogen salt, thereby producing negatively charged silver halide which obviously then has the same type of charge as 1 the colloid carrier. In this manner there are '1 produced emulsions of the protective colloid type, i as contrasted with the emulsions of the present 1 invention in which colloid carrier and sflver hal- 1 ide have opposite charges. In Example 1 of this ,United States patent, the inventor uses a quanitity of silver salt greater than required as a stoichiometric equivalent. But the sequence in *which the various ingredients are caused to react will 'produc 'again a negative charge in the silver halide, and therefore, a charge which would be the same as in the colloid carrier. The sequence of steps in this patent referred to involve first the addition of the halogen salt and subsequently the excess of silver salt in excess. :Th addition of this silver salt at the end of the process and its presence in an excess leads to a formation of a negative charge on the silver halide. This example, therefore, also produces emulsions of the protective colloid type.

Details for carrying out the process are enusimilar steps.

owing to the increase in OH-groups, this acetyl cellulose has particles with strong negative charge, and this charge is further enhanced through the adsorption of additional QH-groups,

due to the alkaline treatment. I

The alkali acetate formed through saponiflcation may easily be removed in a known way either by dialysis or by washing out the flakes of acetyl cellulose precipitated from the treated solution. The formation of silver acetate may be prevented in the further treatment by adding ammonium nitrate'now or at some later stage. to the extent of about 60 grams. This formation of silver acetate also may be prevented byusing solutions of silver nitrate of weak'acidity or by These steps for preventing formation of silver acetate do not interfere with the method of producing th improved photosensitive emulsion.

l The treated solution comprising the acetyl cel-' I lulose after washing in redissolved condition, is now mixed with about 92 cubic centimeters of,a silver, nitrate solution containing 250 grams in 750 cubic centimeters of water. A stoichiometrically equivalent quantity of potassium bromide solution would be 92 cubic centimetersof. a solution containing 175 grams in 750 cubic centimeters of water. Instead of taking this stoichiometrically equivalent quantity, 90 cubic centimeters only of this potassium bromide solution is used. This potassium bromide solution is advisi ably diluted by adding 200 cubic centimeters acetone to prevent precipitation of the acetyl cellu-' lose. While excluding the light, the potassium bromide-acetone solution is now poured in the form or a thin jet and under continuous agitation into the mixture of the silver nitrate and acetyl cellulose.

The result is a grainless thoroughly stable nonsettling emulsion which (without requiring ripening or any washing operations) may be poured directly on any desired carrier, as glass, film,

paper or the like, or which may be employed directly without any carrier in the form of a thin foil. The sensitiveness is the same as of any normalpositive material.

I claim: v f 1. A method of producing a photographic emulsion, which consists of combining with a colloidal carrier whose particles have a charge of a certain polarity, a colloidal silver halide whose particles have a charge of the opposite polarity, maintaining during said combining operation the amount of charged colloidal carrier in such excess relative to the oppositely charged colloidal silver halide that no mutual precipita-' tion occurs, and regulating the combination of the ingredients to produce in the emulsion thus formed a resulting charge of a polarity correspending to that of the colloidal carrier.

2. method oi producing a photographic emulsion, which consists of introducing into a colloidal carrier whose particles have a charge of a certain polarity, a colloidal silver halide whose particles have a charge of the opposite polarity, maintaining during said introduction the amount of charged colloidal carrier in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitation occurs, and regulating the introduction of the colloidal silver halide to produce in the emulsion thus formed a charge deviating from zero and having the same polarity as that of the colloidal carrier.

3. A method of producing a photographic emulsion which consists of producing a colloidal silver halide of a certain polarity within a colloidal carrier whose particles have a charge of the opposite polarity, maintaining during the production of-the silver halide the amount of charged colloidal carrier, in which the silver halide is produced, in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitation occurs, and'regulating the production of the silver halide within the colloidal carrier to produce in the emulsion thus formed a resulting charge of particles different from zero and corresponding to the polarity of the particles of the colloidal carrier.

4. A method of producing a photographic emulsion which consists of producing colloidal silver halide by causing silver salts to react with halide compounds, combining the silver halide thus produced with a colloidal carrier and maintaining during the reaction and combining operation either oneof the reagents of the silver halide in excess of the stoichiometric equivalent or the other reagent of the silver halide, the selection of the excess reagent being such as to give the colloidal silver halide a charge opposite to the polarity of the chargeof the colloidal carrier and to prevent mutual precipitation.

5. A method of producing a photographic emulsion which consists of producing colloidal silver halide by causing a silver salt to react with a halide compound, combining the silver halide with a colloidal carrier whose particles are positively charged and continuously maintaining during the reaction of the silver salt with the halide compound the latter in such excess with respect to the silver salt that though the colloidal silver halide is oppositely charged with respect to the colloidal carrier, no mutual precipitation occurs upon combination of said colloidal silver halide with the said colloidal carrier.

6. A method of producing a photographic emulsion which consists of producing a colloidal silver halide by causing a silver salt to react with a halide compound, combining the colloidal silver halide with a colloidal carrier whose particles are negatively charged, and maintaining during the reaction of the silver salt with the halide compound the silver salt in such excess with respect to the halide compound, that. though the colloidal silver halide be oppositely charged with respect to the colloidal carrier, no mutual precipitation. occurs upon combination of said colloidal silver halide with said colloidal carrier.

7. A method of producing a photographic emulsion which consists of combining with a colloidal carrier whose particles have a charge of a certain polarity, a colloidal silver halide whose particles have a charge of the opposite polarity, maintaining during the combining operation the amount in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitaof charged colloidal carrier tion occurs, while regulating the combination of the ingredients to produce in the emulsion thus formed a resulting charge difierent from zero and. corresponding to the polarity of the colloidal carrier, and adjusting the sensltiveness of the emulsion thus formed by subjecting the emulsion to the action of colloids having a charge of a polarity opposite to that of the resulting charge.

8. A process of producing a photographic emulsion, as set forth in claim 7, wherein the adjustment of the sensltiveness of the emulsion is effected by treating the emulsion with basic dyes when the colloidal carrier has a negative charge.

9. A method of producing a photographic emulsion, as set forth in claim '7, wherein the adjustment of the sensltiveness of the emulsion is efiected by subjecting the emulsion to the action of acidic dyes when the colloidal carrier of the emulsion is positively charged. I

10. A method of producing a photographic emulsion which consists of combining with a colloidal carrier consisting of an incompletely saturated cellulose ester in an organic solvent and whose particles have a charge of a certain polarity, a colloidal silver halide whose particles have a charge of theopposlte polarity maintaining during the combining operation the amount of charged colloidal carrier in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitation occurs, and

regulating the combination of the ingredients to produce in the emulsion thus formed a resulting charge difierent from zero and having the polarity of the cellulose ester solution.

11. A method of producing a photographic emulsion which consists of combining with acetyl cellulose dissolved inacetone and diluted with water and having a charge of certain polarity, a colloidal silver halide whose particles have a charge of the opposite polarity maintaining during the combining operation the amount of charged acetyl cellulose acetone solution in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitation occurs and regulating the combination of the ingredicuts of the emulsion thus formed to produce therein a resulting charge difierent from zero and having the polarity of the acetyl cellulose.

12. A methodof producing a photographic emulsion which consists of modifying the charge of an aqueous solution of cellulose ester by means of an alkali, the treated cellulose ester having particles charged with a certain polarity, combining with the treated cellulose ester a colloidal silver halide whose particles have a charge of the opposite polarity maintaining during the combining operation the amount of the treated cellulose ester in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitation occurs and regulating the combination of the ingredients of the emulsion thus formed to produce therein a resulting charge different from zero and having the polarity of the treated cellulose ester.

13. A. method of producing a photographic emulsion which consists of diluting acetyl cellulose with water, mixing an alkaline solution with the dilutedcellulose to increase the negative charge of the acetyl cellulose, combining with the treated acetyl cellulose acolloidal silver halide whose particles have a positive charge,

' maintaining during the combining operation the amount of the negatively charged acetyl cellulose in such excess relative to the positively charged colloidal silver halide that no mutual precipitation occurs and regulating the combination of the ingredients of the emulsion thus formed to produce therein a-resulting negative charge.

14. A method of producing a photographic I emulsion which consists of modifying the charge of a diluted acetyl cellulose solution by additionof an alkaline solution until the mixture of solu- 1 tions is negatively charged, introducing intothe treated mixture a silver nitratesolution and adding to the mixture of solutions a halide compound of a quantity less than the stoichiometrical j equivalent of the silver nitrate solution while maintaining during the introduction the amount i of the negatively charged mixture of solutions in 1 such excess relative to the oppositely charged silver nitrate solution halide compound that no mutual precipitation occurs to produce an emulsion 01' a charge less than the charge of the treated acetyl cellulose, but of the same polarity.

15. A method of producing a photographic maintaining during the combining operation the amount oi. the charged colloidal carrier in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitation occurs regulating the combination of the ingredients to produce in the emulsion thus formed a resulting charge different from zero and having the polarity ofthe colloidal carrier, and adding a solvent of high boiling point for the colloidal carrier adapted to permit the emulsion to unite with a film layer without dissolving said layer.

' 16. A method of producing a photographic emulsion which consists of modifying the charges of a solution of cellulose ester in water containing organic solvents by means of substances, the

ions of which are absorbed by the cellulose esters,

the treated cellulose ester having particles charged with a certain polarity, combining with the treated cellulose ester a colloidal silver halide whose particles have a charge of the opposite polarity, maintaining during the combining operation the amount of thetreated cellulose ester in such excess relative to the oppositely charged colloidal silver halide that no mutual precipitation occurs, and regulating the combination of the ingredients of the emulsion thus formed to produce therein a resulting charge difierent from zero and having the polarity'of the treated cellulose ester.

FRIEDRICH LIERG. 

